1. Field of the Invention
The present invention relates to an ink-jet printer with the gradation expression power of image enhanced by making the size of ink dots formed on a recording medium variable.
2. Related Art of the Invention
In recent years, with the spread of a personal computer, the demand for a printer, as the output device of a personal computer has increased in leaps and bounds. Especially, in these several years, the occupied ratio of color printers has grown and a speedup and a higher image quality is greatly required for color image printing.
Then, in a conventional ink jet printer, a higher image quality has been implemented by a smaller ink drop ejected from an printer head and a higher dot density on a recording medium. Hereinafter, referring the drawings, one embodiment of piezo type ink ejection using dielectrics will be described.
FIG. 19 shows a conventional ink-jet printer and Numeral 21 denotes a U-shaped flow passage frame, in which a flow passage 22 is formed by closing the opening face with a vibrating plate 25. To this vibrator plate 25, a piezoelectric body 26 with an electrode 27 formed on both sides is fastened and is configured so as to enlarge or contract the volume of the flow passage 22 by the bending of the vibrator plate 25 due to the expansion and contraction of the piezoelectric body 26 under application of a voltage.
On a gradual application of a voltage to the piezoelectric body 26, the piezoelectric body 26 gradually deforms in the outer direction of the flow passage 22 to enlarge the volume of the flow passage 22 and sucks ink into the flow passage 22 through the ink supply port 24. Thereafter, on suddenly removing the applied voltage, the piezoelectric body 26 is suddenly restored to the original shape and the volume of the flow passage 22 is suddenly contracted. As a result, the ink in the flow passage 22 is suddenly pressurized and this pressurized ink is ejected through an ejection port 23 provided on the flow passage frame 21 as ink drops 28.
Besides, as another method for upgrading the image quality, a process of changing the mass or diameter of ink drops is thought of (e.g. Japanese Patent Laid-Open No. 5-261925). Use of a process of changing the diameter of ink drops permits the gradation to be expressed with a change in ink drops ejected from one ejection port, thus enabling images of low-resolution and high image quality to be implemented.
If an attempt is made to implement a high image quality in the former structure of an ink-jet printer, however, it is required to stably eject small ink drops in plenty. This is connected to the upgrade of image resolution and to obtain the same printing speed as formerly, a method such as a increase in the density and number of ejection ports or a speedup of repeated ejections becomes necessary. This increase in the density and number of ejection ports or this speedup of repeated ejections causes a rise in manufacturing cost and a shortening of durable years and is not necessarily satisfied. There was a limit to the high-level compatibility between image quality upgrade and speedup of ink-jet printers.
Also, in a method for changing the mass and diameter of ink drops ejected from the ejection port, no satisfaction is necessarily obtained from the viewpoint of manufacturing, durability or density increase of ejection ports.
Furthermore, a printing head for ink-jet printers disclosed in Japanese Patent Laid-Open No. 5-261925 is based on a method for enlarging the sectional area of the front end in an ejection port and for changing the mass or diameter of ink drops in a simple configuration; however, since a way to change the front end position of ink ejection by use of a certain means and to give the ejection energy to ink by use of another means is employed, it was difficult to eject ink drops of smaller diameter than the size of a small area portion present in the flow passage inside the ejection port and further it was difficult to eject ink of larger diameter than an ejection port of large area.
The present invention intends to solve these problems and its purpose is to provide an ink-jet printer in which the mass or diameter of ink drops is variable corresponding to the need for image quality upgrade and speedup.
The present invention relates to an ink-jet printer for ejecting ink drops from the ejection port. An ink-jet printer according to a first aspect of the present invention comprises a flow passage whose sectional area decreases from the ejection port toward the inside of the ink flow passage, becomes equal to the minimum and then increases further toward the inside of the flow passage, means for drawing the ink front end position inside the flow passage, and means for pushing the ink front end position outside.
Since the present invention is configured so as to change the sectional area of a flow passage from the ejection port to the inside of the flow passage in the sequence of decrease and increase, first, when ejecting small ink drops, the ink front end is pulled from the flow passage of a smallest sectional area to the inner side of a larger sectional area by use of pull-in means and thereafter ink drops are ejected from a portion of a smallest sectional area by use of push-out means, thereby enabling smaller ink drops to be ejected than the sectional area.
Second, when ejecting large ink drops, the ink front end is pulled in to the flow passage of a small sectional area by use of pull-in means and thereafter ink drops are ejected from an ejection port of a larger sectional area by use of push-out means, thereby enabling larger ink drops to be ejected than the sectional area. In this manner, since the mass or the diameter of ink drops can be made larger than the ratio of sectional areas for ejection, an ink-jet printer capable of forming images of a higher gradation is obtained.
Besides, a second configuration of the present invention is such that the wall surface of a flow passage with an increase in sectional area from the portion of the minimum sectional area toward the interior of the flow passage is made hydrophilic.
In this second configuration, since the inner portion from that of the minimum sectional area diameter is made hydrophilic, the front end position of the ink comes at the portion of the minimum sectional area diameter at waiting conditions and then it becomes possible to eject large ink drops by use of means for ejecting drops without pull-in means in case of ejecting large ink drops.
Furthermore, a third configuration of the present invention is such that the wall surface of a flow passage with a decrease in sectional area from the ejection port toward the interior of the flow passage is made water-repellent.
In this third configuration, since the portion of a flow passage with a decrease in sectional area from the ejection port toward the interior of the flow passage is made water-repellant, the front end position of the ink at waiting condition comes at the portion of minimum sectional area and then it also becomes possible to eject large ink drops as the case of the second configuration. Besides, since the portion up to the ejection port is made water-repellant, it becomes possible to reduce the ink drops remaining near the ejection port, thus enabling large ink drops and large ink drops to be stably ejected.